FIGS. 1 and 2 depict conventional magnetic elements 10 and 10′. Such conventional magnetic elements 10/10′ can be used in non-volatile memories, such as magnetic random access memories (MRAM). The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional nonmagnetic spacer layer 16 and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional nonmagnetic spacer layer 16 is conductive. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. The conventional magnetic element 10′ depicted in FIG. 2 is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. However, the conventional barrier layer 16′ is an insulator that is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′. Note that only a single spin valve 10 is depicted, one of ordinary skill in the art will readily recognize that dual spin valves including two pinned layers and two nonmagnetic layers separating the pinned layers from the free layer can be used. Similarly, although only a single spin tunneling junction 10′ is depicted, one of ordinary skill in the art will readily recognize that dual spin tunneling including two pinned layers and two barrier layers separating the pinned layers from the free layer, can be used.
Depending upon the orientations of the magnetization 19/19′ of the conventional free layer 18/18′ and the conventional pinned layer 14/14′, respectively, the resistance of the conventional magnetic element 10/10′, respectively, changes. When the magnetization 19/19′ of the conventional free layer 18/18′ is parallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is low. When the magnetization 19/19′ of the conventional free layer 18/18′ is antiparallel to the magnetization of the conventional pinned layer 14/14′, the resistance of the conventional magnetic element 10/10′ is high.
To sense the resistance of the conventional magnetic element 10/10′, current is driven through the conventional magnetic element 10/10′. Typically in memory applications, current is driven in a CPP (current perpendicular to the plane) configuration, perpendicular to the layers of conventional magnetic element 10/10′ (up or down, in the z-direction as seen in FIG. 1 or 2). Based upon the change in resistance, typically measured using the magnitude of the voltage drop across the conventional magnetic element 10/10′, the resistance state and, therefore, the data stored in the conventional magnetic element 10/10′ can be determined.
It has been proposed that particular materials be used for the conventional magnetic element 10′. In such a conventional magnetic element 10′, it has been proposed that ferromagnetic materials from the group of Ni, Co, and Fe, their alloys such as CoFe, CoFeNi, and low-moment ferromagnetic materials such as CoFeBx, (where x is between five and thirty atomic percent), CoFeC, CoFeHf, or analogous materials be used for the pinned layer 14′ and free layer 18′. In addition, U.S. Patent Application Publication 2005/0040433 (Noziere) proposes the use of certain rare earth-transition metal alloys, such as GdCo, for the free layer 18′. It is known that certain rare earth-transition metal alloys have certain compositions, termed the compensation point, at which the net saturation magnetization becomes zero at a particular temperature. For the conventional barrier layer 16′, it has been proposed that amorphous AlOxor crystalline MgO having (100) or (111) texture be used. For such structures, a large magnetoresistance, up to a several hundred percent difference between the high and low resistance states, can be achieved.
Spin transfer is an effect that may be utilized to switch the magnetizations 19/19′ of the conventional free layers 18/18′, thereby storing data in the conventional magnetic elements 10/10′. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic element 10. The following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. Electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. As a result, a spin-polarized current can switch the magnetization 19′ direction of the conventional free layer 18′ if the current density is sufficiently high (approximately 107-108 A/cm2) and the lateral dimensions of the spin tunneling junction are small (approximately less than two hundred nanometers). In addition, for spin transfer to be able to switch the magnetization 19′ direction of the conventional free layer 18′, the conventional free layer 18′ should be sufficiently thin, for instance, generally less than approximately ten nanometers for Co. Spin transfer based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10/10′ are small, in the range of few hundred nanometers. Consequently, spin transfer is suitable for higher density magnetic memories having smaller magnetic elements 10/10′.
Spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the conventional free layer 18′ of the conventional spin tunneling junction 10′. For example, the magnetization 19′ of the conventional free layer 18′ can be switched from antiparallel to the magnetization of the conventional pinned layer 14′ to parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). The majority electrons traveling from the conventional pinned layer 14′ have their spins polarized in the same direction as the magnetization of the conventional pinned layer 14′. These electrons may transfer a sufficient portion of their angular momentum to the conventional free layer 18′ to switch the magnetization 19′ of the conventional free layer 18′ to be parallel to that of the conventional pinned layer 14′. Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to the magnetization of the conventional pinned layer 14′ to antiparallel to the magnetization of the conventional pinned layer 14′. When current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction), majority electrons have their spins polarized in the direction of magnetization of the conventional free layer 18′. These majority electrons are transmitted by the conventional pinned layer 14′. The minority electrons are reflected from the conventional pinned layer 14′, return to the conventional free layer 18′ and may transfer a sufficient amount of their angular momentum to switch the magnetization 19′ of the free layer 18′ antiparallel to that of the conventional pinned layer 14′.
Although spin transfer can be used in switching the magnetization 19/19′ of the conventional free layer 18/18′, one of ordinary skill in the art will readily recognize that a high current density is typically required. In particular, the current required to switch the magnetization 19/19′ is termed the critical current. As discussed above, the critical current corresponds to a critical current density that is approximately at least 107 A/cm2. One of ordinary skill in the art will also readily recognize that such a high current density implies that a high write current and a small magnetic element size are necessary.
Use of a high critical current for switching the magnetization 19/19′ adversely affects the utility and reliability of such conventional magnetic elements 10/10′ in a magnetic memory. The high critical current corresponds to a high write current. The use of a high write current is associated with increased power consumption, which is undesirable. The high write current may require that larger structures, such as isolation transistors, be used with the conventional magnetic element 10/10′ to form memory cells. Consequently, the areal density of such a memory is reduced. In addition, the conventional magnetic element 10′, which has a higher resistance and thus a higher signal, may be less reliable because the conventional barrier layer 16′ may be subject to dielectric breakdown at higher write currents. Thus, even though a higher signal read may be achieved, the conventional magnetic elements 10/10′ may be unsuitable for use in higher density conventional MRAMs using a high spin transfer switching current to write to the conventional magnetic elements 10/10′. Moreover, although mechanisms for reducing the high current density have been proposed, such as in Noziere, these mechanisms have issues such as a potential loss in signal.
Accordingly, what is needed is a system and method for providing a magnetic memory element that can be switched using spin transfer at a lower write current. The present invention addresses such a need.